Stroke is the third most common cause of death globally after ischemic heart disease and all types of cancer combined. Although it sometimes affects children and young adults, stroke is mainly a disease of older people. Hence by 2020, stroke mortality would have almost doubled, mainly as a result of an increase in the proportion of aging population and the increase in other risk factors (Warlow et al., 2003).
Stroke is defined by WHO as the clinical syndrome of rapid onset of focal (or global) cerebral deficit, lasting more than 24 h or leading to death, with no apparent cause other than a vascular one. There are three pathological types: ischemic stroke, primary intracerebral haemorrhage and subarachnoid haemorrhage (Warlow et al., 2001). In haemorrhagic stroke, blood bursts through the walls of an artery and leaks into the brain (intracerebral haemorrhage) or onto the surface of the brain (subarachnoid haemorrhage).
Ischemic stroke results from a transient or permanent reduction in cerebral blood flow that is restricted to the territory of a major brain artery. The reduction in flow is, in a majority of the cases, due to an occlusion of a cerebral artery either by an embolus or by local thrombosis. Brain tissue is extremely sensitive to ischemia such that even a brief cessation of blood flow to cerebral neurons can initiate a complex sequence of events that ultimately culminate in cellular death. Different brain regions exhibit variable thresholds for ischemia, with white matter being more resilient than grey matter.
Additionally, certain populations of cerebral neurons are selectively vulnerable to ischemia, such as hippocampal CA1 cells and cerebral neurons as compared to dentate granule cells and brain stem neurons respectively (Smith, 2004). Ischemia of cerebral tissue and the ensuing cell death underlie all forms of stroke, including focal ischemia (reduction of nervous system blood supply to focal regions of the brain e.g. Middle cerebral artery (MCA), global ischemia (declining blood flow to entire cerebral hemisphere/s) and possibly, intraparenchymal hemorrhage (Karpiak et al, 1989; Smith, 2004).
Within minutes of focal vascular occlusion, brain tissue is deprived of glucose and oxygen, resulting in the accumulation of acidic by-products of metabolism. This loss of substrate and drop in pH leads to a cessation of the electron transport chain activity in the mitochondria and a rapid decline in ATP concentration. Loss of ATP eventually leads to failure of the Na+/K+-ATPase, which results in a marked increase of intracellular Na+. Persistent depolarization allows Ca2+ entry whilst a higher intracellular Na+ concentration reduces the efficacy of the 2Na+/Ca2+ symporter, further increasing intracellular Ca2+ (Smith, 2004). Since membrane potential reaches the electrical threshold for discharge, neurons fire repetitively and release their neurotransmitters locally and at distant targets. Accumulation of glutamate in the extracellular space results in the activation of NMDA and metabotropic glutamate receptors, contributing to Ca2+ overload. As a consequence of glutamate-mediated overactivation, Na+ and Cl− enter neurons via channels for monovalent ions (eg. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors). Water follows passively down an osmotic gradient and the ensuing edema can affect perfusion and give rise to increased intracranial pressure, vascular compression and herniation (Dirnagl et al, 1999). In fact, brain edema is one of the major determinants as to whether a patient survives beyond the first few hours after stroke.
An increase in the universal second messenger, Ca2+, initiates a series of cytoplasmic and nuclear events that impact the development of tissue damage profoundly, such as activation of proteolytic enzymes that degrade cytoskeletal proteins like actin and spectrin (Furukawa et al, 1997), as well as extracellular matrix proteins like laminin (Chen and Strickland, 1997). Activation of cytosolic PLA2 and cyclooxygenase generates free radical species that overwhelm endogenous scavenging mechanisms, producing lipid peroxidation and membrane damage (Zhao et al, 1994; Weisbrot-Lefkowitz et al, 1998). The importance of oxygen free radicals in cell damage associated with stroke is highlighted by the fact that even delayed treatment with free radical scavengers can be effective in experimental focal ischemia (Zhao et al, 1994).
Oxygen free radicals also serve as important signalling molecules that trigger inflammation. Mediators of inflammation, such as platelet-activating factor, TNF-a and IL-1β are produced by injured brain cells (Rothwell and Hopkins, 1995). Consequently, the expression of adhesion molecules on the endothelial cells surfaces is induced, promoting the migration of neutrophils into brain parenchyma. Macrophages and monoctyes follow, becoming the predominant cells in the ischemic brain five to seven days after the occurrence of stroke (Iadecola, 1997). Chemokines like IL-8 are produced by the injured brain and play key roles in chemotaxis of blood-borne inflammatory cells. Post-ischemic inflammation can contribute to ischemic damage by many mechanisms. The haemodynamic, metabolic and ionic changes described above do not affect the ischemic territory homogenously in the case of focal cerebral ischemia. Here, blood flow is most greatly reduced in a central region of the brain, known as the core, and in a graded manner centrifugally from the core, an area termed the penumbra (Hakim, 1987). Cerebral blood flow decreases to less than 15% of baseline within the core, whilst it is between 15% to 40% of baseline in penumbral regions. All neurons in the core will infarct if duration of ischemia is thirty minutes or more whereas only some in the penumbra will die, depending on the length of ischemia. By definition, the penumbra is that ischemic region that is functionally impaired but viable and potentially salvageable with timely therapeutic intervention (Schaller and Graf, 2004; Fisher, 2004). Hence, the process by which the penumbra is destroyed is the focus of most ischemic research as prevention of this infarct growth would be expected to rescue neuronal tissue.
Two strategies exist for reducing nerve cell death after ischemic insult: (a) insult limitation whereby blood flow is restored before infarction can occur (reperfusion) and (b) neuroprotection, which involves intervening to reduce specific mechanisms responsible for neuronal death thereby reducing the brain's intrinsic vulnerability to a given insult (Choi, 2000). A good example of insult limitation is the direct use of tissue plasminogen activator (tPA) to dissolve brain arterial blood clots. If accomplished within 3 hr of the onset of stroke symptoms, tPA is effective in improving clinical outcome (The National Institute of Neurological Disorders and Stroke rtPA Stroke Study Group, 1995) and is currently the only approved stroke therapy. To this end, defibrinogenating agents such as ancrod (Sherman et al, 2000) and thrombolytic agents like purokinase (Furlan et al, 1999) have been found to be capable of initiating reperfusion after ischemia, though neither has achieved regulatory approval.
Neuroprotection, on the other hand, is based on identifying and then blocking specific mechanisms involved in ischemic cell death. Two modes of cell death predominate after stroke, namely, necrosis and apoptosis. Necrosis takes place primarily in the core whilst apoptosis defines death in the penumbra or any region with less severe declines in cerebral blood flow (Ye et al, 2001). All previously performed pivotal trials of neuroprotective drugs have, however, failed to achieve a significant treatment effect on the pre-specified primary outcome measure (Fisher and Schaebitz, 2000).
Whilst the strategy of constraining cerebral ischemia through restoration of blood flow has almost reached the clinical trials, the identification of alternative and/or improved neuroprotective agents will be useful.
Currently, the only approved acute stroke therapy is intravenous recombinant tPA (tissue plasminogen activator) administration initiated within three hours of stroke onset. However, emerging data suggests that under some conditions, tPA can be potentially neurotoxic (Tsirka et al, 1997; Wang et al, 1998; Nagai et al, 2001). As such, there is a need to encompass newer approaches to intervention in this field of medicine.